text: #include <tommath.h>
  #ifdef BN_MP_PRIME_RANDOM_EX_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis
   *
   * LibTomMath is a library that provides multiple-precision
   * integer arithmetic as well as number theoretic functionality.
   *
   * The library was designed directly after the MPI library by
   * Michael Fromberger but has been written from scratch with
   * additional optimizations in place.
   *
   * The library is free for all purposes without any express
   * guarantee it works.
   *
   * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com
   */
  
  /* makes a truly random prime of a given size (bits),
   *
   * Flags are as follows:
   * 
   *   LTM_PRIME_BBS      - make prime congruent to 3 mod 4
   *   LTM_PRIME_SAFE     - make sure (p-1)/2 is prime as well (implies LTM_PRIME_BBS)
   *   LTM_PRIME_2MSB_OFF - make the 2nd highest bit zero
   *   LTM_PRIME_2MSB_ON  - make the 2nd highest bit one
   *
   * You have to supply a callback which fills in a buffer with random bytes.  "dat" is a parameter you can
   * have passed to the callback (e.g. a state or something).  This function doesn't use "dat" itself
   * so it can be NULL
   *
   */
  
  /* This is possibly the mother of all prime generation functions, muahahahahaha! */
  int mp_prime_random_ex(mp_int *a, int t, int size, int flags, ltm_prime_callback cb, void *dat)
  {
     unsigned char *tmp, maskAND, maskOR_msb, maskOR_lsb;
     int res, err, bsize, maskOR_msb_offset;
  
     /* sanity check the input */
     if (size <= 1 || t <= 0) {
        return MP_VAL;
     }
  
     /* LTM_PRIME_SAFE implies LTM_PRIME_BBS */
     if (flags & LTM_PRIME_SAFE) {
        flags |= LTM_PRIME_BBS;
     }
  
     /* calc the byte size */
     bsize = (size>>3) + ((size&7)?1:0);
  
     /* we need a buffer of bsize bytes */
     tmp = OPT_CAST(unsigned char) XMALLOC(bsize);
     if (tmp == NULL) {
        return MP_MEM;
     }
  
     /* calc the maskAND value for the MSbyte*/
     maskAND = ((size&7) == 0) ? 0xFF : (0xFF >> (8 - (size & 7)));
  
     /* calc the maskOR_msb */
     maskOR_msb        = 0;
     maskOR_msb_offset = ((size & 7) == 1) ? 1 : 0;
     if (flags & LTM_PRIME_2MSB_ON) {
        maskOR_msb       |= 0x80 >> ((9 - size) & 7);
     }  
  
     /* get the maskOR_lsb */
     maskOR_lsb         = 1;
     if (flags & LTM_PRIME_BBS) {
        maskOR_lsb     |= 3;
     }
  
     do {
        /* read the bytes */
        if (cb(tmp, bsize, dat) != bsize) {
           err = MP_VAL;
           goto error;
        }
   
        /* work over the MSbyte */
        tmp[0]    &= maskAND;
        tmp[0]    |= 1 << ((size - 1) & 7);
  
        /* mix in the maskORs */
        tmp[maskOR_msb_offset]   |= maskOR_msb;
        tmp[bsize-1]             |= maskOR_lsb;
  
        /* read it in */
        if ((err = mp_read_unsigned_bin(a, tmp, bsize)) != MP_OKAY)     { goto error; }
  
        /* is it prime? */
        if ((err = mp_prime_is_prime(a, t, &res)) != MP_OKAY)           { goto error; }
        if (res == MP_NO) {  
           continue;
        }
  
        if (flags & LTM_PRIME_SAFE) {
           /* see if (a-1)/2 is prime */
           if ((err = mp_sub_d(a, 1, a)) != MP_OKAY)                    { goto error; }
           if ((err = mp_div_2(a, a)) != MP_OKAY)                       { goto error; }
   
           /* is it prime? */
           if ((err = mp_prime_is_prime(a, t, &res)) != MP_OKAY)        { goto error; }
        }
     } while (res == MP_NO);
  
     if (flags & LTM_PRIME_SAFE) {
        /* restore a to the original value */
        if ((err = mp_mul_2(a, a)) != MP_OKAY)                          { goto error; }
        if ((err = mp_add_d(a, 1, a)) != MP_OKAY)                       { goto error; }
     }
  
     err = MP_OKAY;
  error:
     XFREE(tmp);
     return err;
  }
  
  
  #endif
  
  /* $Source: /cvs/libtom/libtommath/bn_mp_prime_random_ex.c,v $ */
  /* $Revision: 1.4 $ */
  /* $Date: 2006/03/31 14:18:44 $ */
  